The present invention relates to a method for operating an internal combustion engine with an exhaust gas purification system, in which a rich-burn mode of the internal combustion engine with a rich exhaust gas composition or a lean-burn mode of the internal combustion engine with a lean exhaust gas composition is set alternately, and in which the quantity of fuel which is introduced during a combustion cycle is introduced by means of at least one preinjection, at least one main injection and at least one afterinjection.
The present invention also relates to an internal combustion engine with exhaust gas purification system for carrying out the method according to the invention.
Nitrogen oxides in oxygen-rich combustion exhaust gases can be very successfully reduced to nitrogen with the aid of ammonia. In this process, it is appropriate for the ammonia to be generated in a catalytic converter unit in the vehicle itself. The ammonia-generating catalytic converter unit may in this case be formed, for example, by a three-way catalytic converter which, with a substoichiometric exhaust gas composition, synthesizes ammonia NH3 from nitrogen oxides NOx and hydrogen H2. The ammonia is stored in a downstream nitrogen oxide reduction catalytic converter when the exhaust gas composition is substoichiometric or rich. When the exhaust gas composition subsequently becomes superstoichiometric or lean, this ammonia reduces nitrogen oxides to nitrogen in the nitrogen oxide reduction catalytic converter. The quantity of ammonia which is generated is dependent on the quantity of nitrogen oxides provided during substoichiometric or rich combustion.
The term lean-burn mode is to be understood as meaning a superstoichiometric engine operating mode, in which an excess of oxygen, i.e. λ>1, is present during the combustion. The term rich-burn mode is to be understood as meaning a substoichiometric engine operating mode, in which an excess of fuel, i.e. λ<1, is present during the combustion. Accordingly, a lean exhaust gas composition denotes an excess of oxygen in the exhaust gas, and a rich exhaust gas composition denotes an excess of fuel in the exhaust gas.
German laid-open specification DE 198 20 828 A1 discloses an exhaust gas purification system which during a rich exhaust gas composition uses a first catalytic converter unit to generate ammonia from hydrogen and nitrogen oxides contained in the exhaust gas and stores the ammonia which is generated by means of a second catalytic converter unit. When the exhaust gas composition is lean, nitrogen oxides which are contained in the exhaust gas are subjected to a reduction reaction using the ammonia which has been temporarily stored as reducing agent. To ensure also that sufficient nitrogen oxides are contained in the exhaust gas when the exhaust gas composition is rich to synthesize a significant quantity of ammonia, a third catalytic converter unit, which temporarily stores nitrogen oxides contained in the exhaust gas when the exhaust gas composition is lean and releases the previously stored nitrogen oxides again when the exhaust gas composition is rich, is connected upstream of the first catalytic converter unit, which is provided for the generation of ammonia when the exhaust gas composition is rich.
German patent DE 197 50 226 C1 discloses an engine control unit for a diesel engine with an exhaust gas purification system which is provided with an adsorption device for nitrogen oxides, NOx, which adsorbs nitrogen oxides when the exhaust gas composition is lean. A rich exhaust gas composition in which the exhaust gases have a reducing atmosphere is necessary from time to time to regenerate the absorber system. To set a rich or lean exhaust gas composition, a fuel quantity is injected by a preinjection, a main injection and a fuel afterinjection. The fuel afterinjection is substantially merely evaporated and treated, but only burnt in a small proportion, resulting in a very high emission of unburnt hydrocarbons HC and carbon monoxide CO. If the adsorption device releases the stored nitrogen oxides when the exhaust gas composition is rich, the reaction products hydrocarbon HC and carbon monoxide CO on the adsorber surfaces serve to convert nitrogen oxides into nitrogen N2.
German laid-open specification DE 100 29 504 A1 discloses a multi-stage combustion method for diesel engines, in which an air/fuel ratio λ for the fuel to be burnt and the combustion air supplied is set according to predetermined values by a control unit. A control unit can be switched from normal lean-burn mode to a temporary rich-burn mode. An afterinjection of fuel which is set back in terms of time with respect to the main injection and, if appropriate, any desired number of preinjections are provided for in the rich-burn mode. Superstoichiometric or substoichiometric air/fuel ratios can be created as required by means of a further afterinjection which is set back in terms of time. The combustion method is suitable for providing appropriate exhaust gas compositions and temperatures for the regeneration of NOx adsorber systems in the rich-burn mode.
This invention is intended to provide a method for operating an internal combustion engine and also an internal combustion engine in which a quantity of nitrogen oxides which is sufficient for the synthesis of significant quantities of ammonia is provided, even with a rich exhaust gas composition, by influencing the combustion of fuel in the internal combustion engine.
For this purpose, the invention provides a method for operating an internal combustion engine with exhaust gas purification system, in which a rich-burn mode of the internal combustion engine with a rich exhaust gas composition or a lean-burn mode of the internal combustion engine with a lean exhaust gas composition is set alternately, and in which the quantity of fuel which is introduced during a combustion cycle of the internal combustion engine is introduced by means of at least one preinjection, at least one main injection and at least one afterinjection. In the rich-burn mode a preinjection of fuel takes place at an early time, in order to achieve a prehomogenized mix in the combustion chamber, the quantity of fuel injected by the preinjection is set in such a way, and a combustion chamber pressure is lowered by means of intake air throttling in such a way, that a mix which is not initially ignitable is present in the combustion chamber, a main injection is added to the prehomogenized mix in the combustion chamber in order to improve the ignitability of the mix which is present in the combustion chamber, combustion of the at least one preinjection and of the at least one main injection is carried out under an excess of oxygen, and a rich exhaust gas composition is set by means of the at least one afterinjection.
High temperatures and therefore large quantities of nitrogen oxide with significantly low levels of exhaust gas blackening are formed as a result of the homogenized fuel preparation and the combustion of the virtually completely homogenous cylinder charge under excess oxygen. The substoichiometric exhaust gas is achieved by a set-back afterinjection which takes some part in the combustion. As a result, even in rich-burn mode sufficient nitrogen oxides are made available to synthesize a significant quantity of ammonia. An additional NOx adsorber connected upstream of the ammonia-generating catalytic converter can consequently be made smaller or, if appropriate, even omitted altogether. Since a significant quantity of nitrogen oxide is provided even in the rich-burn mode, it is possible to reduce the duration of the rich-burn mode which is required for regeneration of the catalytic converter, with the result that the fuel consumption drops. The combustion method according to the invention allows high exhaust gas temperatures as are required for the synthesis of ammonia from nitrogen and hydrogen in an ammonia-generating catalytic converter to be generated even in rich-burn mode.
The intake air throttling is carried out in such a manner that during the main combustion the air/fuel ratio in the combustion chamber is lower than 2.0. The prehomogenization of the fuel/air mix resulting from the injection strategy which is selected in the invention means that the main combustion takes place at low air/fuel ratios and a high combustion chamber temperature, with the result that a high concentration of nitrogen oxides is formed in the combustion chamber. The maximum nitrogen oxide concentration is achieved at an air/fuel ratio of from 1.1 to 1.3. Premature or uncontrolled spontaneous ignition of the cylinder charge before top dead center is in this case prevented by the intake air throttling which is carried out, leading to a drop in the combustion chamber pressure. The prehomogenized cylinder charge is therefore only ignited by the targeted main injection in the region of the top dead center and burns at a high temperature. As has already been stated above, the air/fuel ratio selected results in high nitrogen oxide emissions combined, at the same time, with a low level of exhaust gas blackening.
It is advantageous for there to be no exhaust gas recirculation in rich-burn mode.
According to one feature of the invention a controllable inlet swirl flow is generated.
A controllable inlet swirl flow assists with the prehomogenization of the fuel/air mix in the combustion chamber and the intake air throttling.
According to another feature of the invention, the intake air throttling is effected by means of at least one throttle valve and/or a variable inlet valve control.
By way of example, each cylinder may be assigned a throttle valve, which is then arranged downstream of the exhaust gas recirculation device. As an alternative to one throttle for each cylinder, it is also possible to provide throttles for in each case a cylinder bank or a plurality of cylinders. As an alternative or in addition to throttle valves, it is also possible to provide a variable inlet valve control. Alternatively, the intake air throttling device may also be arranged upstream of the exhaust gas recirculation device, as seen in the direction of flow. All the means for intake air throttling are actuated by an actuating drive, which is actuated by auxiliary forces, as a function of signals from an electronic engine control unit.
According to another feature of the invention, a controllable inlet swirl flow is generated.
The dispersion of the fuel injected during the compression phase is promoted by a high swirl flow and boosts the effect whereby the prehomogenized mix is not ignited prematurely or in an uncontrolled manner.
According to another feature of the invention, the at least one preinjection and the at least one main injection are burnt at a λ value of less than 2 and preferably between 1.1 and 1.3.
In this way, a sufficient quantity of nitrogen oxide for the synthesis of ammonia is formed even in the rich-burn mode.
According to another feature of the invention, the at least one main injection is introduced into the combustion chamber in the region of the top dead center.
The at least one main injection triggers the ignition of the fuel/air mix which is present in the combustion chamber or at least considerably improves its ignitability. The main injection may in this case take place in a range from approximately 10° before TDC to 10° after TDC, preferably in a range from 4° before TDC to 4° after TDC.
According to another feature of the invention, the preinjection is introduced into the combustion chamber even before the bottom dead center.
The prehomogenization can be boosted in this way. Oil dilution is avoided by injection in the seat throttle region.
The problem on which the invention is based is also solved by an internal combustion engine for carrying out the method according to the invention, having a controllable injection system in which means are provided for prehomogenizing the quantity of fuel introduced by means of at least one preinjection and means are provided for intake air throttling.
Further features and advantages of the invention will emerge from the claims and the following description of a preferred embodiment of the invention with reference to the drawings.